Few sighted people fully understand the great difference between 2D tactile and 2D visual perception. It is far more difficult for a blind person's fingers and hands to provide him/her with the kind of overview of a tactile picture that sighted people perceive instantly with their eyes. The human eye and its associated parts of the brain can take in and process enormously more information in a moment than can be perceived tactilely in hours.
Most people who are born blind or who lose their sight at an early age have great difficulty understanding information presented in 2D tactile pictures. Sighted children have to learn about parallax, representation of 3D objects by 2D projections, and use of spatial position in such things as maps and graphs. Blind children seldom have access to comparable tactile pictures and do not develop these concepts. As a consequence, blind students often have great difficulty with topics such as geometry and function graphs that are typically taught using 2D pictures.
The best advice known to this author on how to use and not use tactile graphics was given by Ms. Cathy Mack, a former special education teacher and expert on technologies for blind children. An excerpt from her contribution to a symposium on tactile graphics[7] follows:
When you introduce a tactile diagram, whether it's to a kindergartner or to an adult, it's important that it be presented in an informational way, not as a guessing game. Many times I've seen a person hand a tactile diagram to someone and say ``can you tell what this is?'' You have to keep in mind that a diagram done by a Braille printer, 11 by 11 inch size, whether a diagram of a cat, or a diagram of the Empire State building is going to occupy roughly the same amount of space on that paper. So, the person approaching that picture tactilely doesn't have a clue about the frame of reference. So it's rarely productive to say ``can you tell what this is?'' It makes so much more sense to say, ``This is a diagram of a church. You'll see the pointed steeple at the top'' or one little bit of information to give the person a starting point to get some meaning from that picture.